The present invention relates to noise reduction in a system comprising a digitizer and, more particularly, but not exclusively to noise reduction in a system comprising a digitizer associated with a display screen.
U.S. Pat. No. 6,690,156 “Physical Object Location Apparatus and Method and a Platform using the same” assigned to N-trig Ltd, and U.S. patent application Ser. No. 10/649,708 “Transparent Digitizer” also assigned to N-trig Ltd, describe a positioning device capable of detecting multiple physical objects, preferably styluses, located on a flat screen display. One of the preferred embodiments in both patents describes a system built of transparent foils containing a matrix of vertical and horizontal conductors. In one embodiment the stylus includes a passive resonance circuit, which is triggered by an excitation coil that surrounds the foils. The stylus is excited at a predetermined range of frequencies depending on the capacitance and inductance of the resonant circuit. Other embodiments may include a different kind of EM stylus. The exact position of the stylus is determined by processing the signals that are sensed by the matrix of horizontal and vertical conductors.
Existing digitizer systems use several noise removal methods to improve the detection precision. For example the received signal is processed through a band pass filter leaving a window of frequencies including the stylus frequency. The filtered signal may then be passed through a Fourier transform selecting the single frequency of the stylus.
Elements that induce an equal amount of noise on each conductive line regardless of the line location may then be eliminated through the use of differential amplifiers. For example, objects that are far enough from the sensor will have the same effect on all the sensor lines.
There are other examples of noise reduction methods that do not eliminate noise at the stylus frequency.
Using the various prior art systems, much of the noise is removed, but one element of noise necessarily remains because it cannot be identified and filtered out and that is noise that is at the same frequency as the stylus.
Preferred Application
The preferred application to which the embodiments to be described hereinbelow are applicable is a transparent digitizer for a mobile computing device that uses a flat panel display (FPD) screen. The digitizer detects the position of one stylus at a very high resolution and update rate. The stylus is used for pointing, painting, writing (hand write recognition) and any other activity that is typical for a stylus. The digitizer supports full mouse emulation. As long as the stylus hovers above the FPD, a mouse cursor follows the stylus position. Touching the screen stands for left click and a special switch located on the stylus emulates right click operation.
The application may utilize a passive EM stylus. External excitation coils that surround the sensor are utilized to energize the stylus. However, other versions may include an active stylus, battery operated or wire connected, which does not require external excitation circuitry.
In one application the electromagnetic object responding to the excitation is a stylus. However, other embodiments may include other physical objects comprising a resonant circuit or active oscillators, such as gaming pieces. Applications describing gaming tokens comprising resonant circuits are described in U.S. Pat. No. 6,690,156 (“physical object location apparatus and method and a platform using the same”).
In the preferred application, the digitizer can detect simultaneous and separate inputs from an electromagnetic stylus and a user finger. Hence, it is capable of functioning as a touch detector as well as detecting a stylus. However, other embodiments may include a digitizer capable of detecting only an electromagnetic stylus.
In a preferred application, the stylus supports full mouse emulation. However, in different applications the stylus could support additional functionality such as an Eraser, change of color, etc. In other embodiments the stylus could be pressure sensitive and changes its frequency or changes other signal characteristics in response to user pressure.
In a preferred application, the mobile device is an independent computer system having its own CPU. In different embodiments the mobile device might only be a part of system such as a wireless mobile screen for a Personal Computer.
In a preferred application, the digitizer is integrated into the host device on top of the FPD screen. In additional application the transparent digitizer can be provided as an accessory that could be placed on top of a screen. Such a configuration can be very useful for laptop computers, which are already in the market in very large numbers. Such systems can turn a laptop into a powerful device that supports hand writing, painting or any other operation enabled by the transparent digitizer.
In a preferred application, the digitizer supports one stylus. However, in different applications more than one stylus may operate simultaneously on the same screen. Such a configuration is very useful for entertainment application where multiple users can paint or write to the same paper-like screen.
In one application, the digitizer is implemented on a set of transparent foils. Alternatively such a digitizer may be implemented using either a transparent or a non-transparent sensor. One example is a Write Pad device, which is a thin digitizer that is placed below normal paper. In this example, the stylus combines real ink with electro magnetic functionality. The user writes on the normal paper and the input is simultaneously transferred to a host computer to store or analyze the data.
An additional example of a non-transparent sensor is an electronic entertainment board. The digitizer, in this example, is mounted below the graphic image of the board, and detects the position and identity of gaming figures that are placed on top the board. The graphic image in this case is static, but it could be manually replaced from time to time (such as when switching to a different game).
In some applications a non-transparent sensor could be integrated in the back of a FPD. One example for such an embodiment is an electronic entertainment device with a FPD display. The device could be used for gaming, in which the digitizer detects the position and identity of gaming figures. It could also be used for painting and/or writing in which the digitizer detects one ore more styluses. In most cases, a configuration of non-transparent sensor with a FPD will be used when high performance is not critical for the application.